The present invention relates to a process for the production of aqueous inks, free of foreign or extrinsic salts, from technical dyes.
In ink-jet printing methods, a printing or ejection head which is supplied with ink from an ink reservoir, transfers ink in droplet form onto a print carrier. Short-term or temporary pressures are generated by a ceramic oscillator in the cavities of this printing head, and these pressures lead to the ejection of ink from capillary nozzles. A clean printed image requires droplets of equal size. These droplets must be applied to the print carrier in a grid pattern. Among the factors influencing production of droplets of equal size are the nozzle geometry and the configuration of the break-off plane, as well as the wetting behavior of the ink within and/or outside of the nozzles. To stabilize the ink meniscus in the nozzles after breakup of the droplets and to form the droplets, the ink must exhibit a surface tension which is as high as possible.
The ink is supplied into the nozzles of the ink-jet printing head, in part, by way of capillary cross sections, and will normally continue to flow without the application of pressure. In order to attain a flow of ink in sufficient amounts to achieve a high droplet sequence, and thus a high ejection speed, the ink must have a low viscosity, preferably around 2 cp. During relatively long periods of inactivity between printing periods, ink components must not crystallize out in the area of the printing head and the ink components must not colloidally segregate from the ink. Moreover, there must not be any surface film formation in the nozzle opening as a result of evaporation of water.
It is important that the ink does not attack the materials which make up the printing head. Since the printing head may comprise, for example, individual small steel plates which are bonded to one another into a package by a diffusion welding process, the ink should not attack steel or the intermediate, diffusion welding layers.
A number of inks have been disclosed for use, for example, in printing methods employing ink-jet printing. These inks, however, do not always completely satisfy the required ink parameters.
Anionic dyes of high water solubility are used for the manufacture of inks for use in ink-jet printers of the type discussed hereinabove. Such technical dyes are usually available in the form of sulfonic acid sodium salts, and generally contain fairly large amounts of foreign salts, such as NaCl, Na.sub.2 SO.sub.4 and Na.sub.2 CO.sub.3. These salts are unavoidably contained in the dyes due to the manufacturing method of the dyes, or they are added to the dyes as processing aids. The salts increase the risk of corrosion by the ink and the tendency of the ink to crystallize. Additionally, the solubility of the dye in auxiliary solvents is greatly reduced due to the presence of these salts.
An aqueous black ink suitable for a printing method where the ink is transferred in droplets onto a print carrier along with a method for its manufacture are disclosed in U.S. Pat. No. 4,159,203. As the dye salts, two direct dyes are used which are not black. These dyes form a genuine or true solution with a solvent, for example, formamide. While the dyes are completely dissolved in the mixture, the foreign salts, obtained in the production of the dye or also added subsequently, such as sodium sulfate and/or sodium carbonate, can be removed by coarse filtration. The sodium chloride salt, soluble up to 9 g per 100 g of solvent, dissolves completely, if the solution is stirred too much. In the case of insufficient stirring, a large amount of the dyes will not dissolve, so that the provided concentration of dye in the ink will not be obtained. A concentration of the filtrate by distillation is difficult due to the high boiling point of formamide (212.degree. C.), so that formamide is left as the solvent in the ink to be manufactured. As a result of this presence of formamide, the number of different ink versions is greatly restricted.